The present invention relates to a grain drying and conditioning apparatus of a continuous flow type having air inlet, grain inlet, grain outlet and spaced air pervious walls for confining a column of grain to be dried. A blower and burner mechanism is also connected to the apparatus for causing heated air to be forced into a plenum chamber inside the inner pervious wall and through the column of grain to heat and extract moisture therefrom.
In a typical crossflow column grain dryer discussed by Litchfield & Zhang, ASEA 936511, typically a "drying front" moves through the mass in the column as the material moves through the dryer. There is a relationship between the temperature of the product mass in a column and moisture removal. It has long been known that when the drying front has passed completely through a column there will be a simultaneous increase in the temperature of the exhaust air because of decreased evaporative cooling of the air.
Studies show that corn drying in a typical crossflow dryer column is a three-phase process: start-up heating, dominant drying, and tail-drying heating phases. Two-step temperature increase was caused by uneven energy distribution drying at different phases of the drying. At first, wet corn needed to be heated to a certain level to build a temperature gradient within kernels to generate moisture migration force. During this period, most of the energy supplied by the drying-air was used to warm up the corn, and a rapid increase of corn temperature resulted. This period was the start-up heating phase in which the corn mass underwent a significant temperature increase and a slight moisture loss.
After the temperature gradient in kernels was built up, a constant rate of moisture migration could be generated. The point undergoing the first significant temperature increase was defined as the drying front. The drying front indicated the start of the dominant drying phase, in which most of the heat carried by drying-air was used to evaporate moisture from kernels, and the corn mass underwent a significant moisture loss and a relatively small temperature increase.
The drying front always begins in the column area closest to the drying air source.
When the amount of moisture that could be migrated from kernels was less than could be carried by the drying-air, the moisture removal rate was slowed down. Most of the heat was again used to heat corn, and caused another rapid increase in corn mass temperature. The point undergoing the second significant temperature increase was defined as the end of the drying front. Too much time in the dryer after the dominant drying phase can cause overheating and overdrying of the grain.
The end of drying occurs first in the column area closest to the drying air source, and therefore overheating of grain occurs first in this area.
It is also known that conventional continuous flow column grain drying devices are limited in their maximum plenum temperatures and therefore in their drying efficiency because of the kernel temperature limits of the layer of grain that is continuously exposed to the hot plenum air as the grain moves downward sliding against the inner perforated grain column wall that forms the plenum chamber walls, particularly after the end of the drying front. There is, therefore, a need for equipment of this type which will overcome this kernel temperature limitation and improve drying efficiency by providing a design that limits the amount of time that a given layer of grain can move in direct contact with the heat plenum wall. Also, it allows dry grain to move out of the dryer without being overdried.